fmemopen(3) — Linux manual page


FMEMOPEN(3)               Linux Programmer's Manual              FMEMOPEN(3)

NAME         top

       fmemopen -  open memory as stream

SYNOPSIS         top

       #include <stdio.h>

       FILE *fmemopen(void *buf, size_t size, const char *mode);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:

DESCRIPTION         top

       The fmemopen() function opens a stream that permits the access
       specified by mode.  The stream allows I/O to be performed on the
       string or memory buffer pointed to by buf.

       The mode argument specifies the semantics of I/O on the stream, and
       is one of the following:

       r      The stream is opened for reading.

       w      The stream is opened for writing.

       a      Append; open the stream for writing, with the initial buffer
              position set to the first null byte.

       r+     Open the stream for reading and writing.

       w+     Open the stream for reading and writing.  The buffer contents
              are truncated (i.e., '\0' is placed in the first byte of the

       a+     Append; open the stream for reading and writing, with the
              initial buffer position set to the first null byte.

       The stream maintains the notion of a current position, the location
       where the next I/O operation will be performed.  The current position
       is implicitly updated by I/O operations.  It can be explicitly
       updated using fseek(3), and determined using ftell(3).  In all modes
       other than append, the initial position is set to the start of the
       buffer.  In append mode, if no null byte is found within the buffer,
       then the initial position is size+1.

       If buf is specified as NULL, then fmemopen() allocates a buffer of
       size bytes.  This is useful for an application that wants to write
       data to a temporary buffer and then read it back again.  The initial
       position is set to the start of the buffer.  The buffer is
       automatically freed when the stream is closed.  Note that the caller
       has no way to obtain a pointer to the temporary buffer allocated by
       this call (but see open_memstream(3)).

       If buf is not NULL, then it should point to a buffer of at least len
       bytes allocated by the caller.

       When a stream that has been opened for writing is flushed (fflush(3))
       or closed (fclose(3)), a null byte is written at the end of the
       buffer if there is space.  The caller should ensure that an extra
       byte is available in the buffer (and that size counts that byte) to
       allow for this.

       In a stream opened for reading, null bytes ('\0') in the buffer do
       not cause read operations to return an end-of-file indication.  A
       read from the buffer will indicate end-of-file only when the current
       buffer position advances size bytes past the start of the buffer.

       Write operations take place either at the current position (for modes
       other than append), or at the current size of the stream (for append

       Attempts to write more than size bytes to the buffer result in an
       error.  By default, such errors will be visible (by the absence of
       data) only when the stdio buffer is flushed.  Disabling buffering
       with the following call may be useful to detect errors at the time of
       an output operation:

           setbuf(stream, NULL);

RETURN VALUE         top

       Upon successful completion, fmemopen() returns a FILE pointer.
       Otherwise, NULL is returned and errno is set to indicate the error.

VERSIONS         top

       fmemopen() was already available in glibc 1.0.x.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface   Attribute     Value   │
       │fmemopen(), │ Thread safety │ MT-Safe │

CONFORMING TO         top

       POSIX.1-2008.  This function is not specified in POSIX.1-2001, and is
       not widely available on other systems.

       POSIX.1-2008 specifies that 'b' in mode shall be ignored.  However,
       Technical Corrigendum 1 adjusts the standard to allow implementation-
       specific treatment for this case, thus permitting the glibc treatment
       of 'b'.

NOTES         top

       There is no file descriptor associated with the file stream returned
       by this function (i.e., fileno(3) will return an error if called on
       the returned stream).

       With version 2.22, binary mode (see below) was removed, many
       longstanding bugs in the implementation of fmemopen() were fixed, and
       a new versioned symbol was created for this interface.

   Binary mode
       From version 2.9 to 2.21, the glibc implementation of fmemopen()
       supported a "binary" mode, enabled by specifying the letter 'b' as
       the second character in mode.  In this mode, writes don't implicitly
       add a terminating null byte, and fseek(3) SEEK_END is relative to the
       end of the buffer (i.e., the value specified by the size argument),
       rather than the current string length.

       An API bug afflicted the implementation of binary mode: to specify
       binary mode, the 'b' must be the second character in mode.  Thus, for
       example, "wb+" has the desired effect, but "w+b" does not.  This is
       inconsistent with the treatment of mode by fopen(3).

       Binary mode was removed in glibc 2.22; a 'b' specified in mode has no

BUGS         top

       In versions of glibc before 2.22, if size is specified as zero,
       fmemopen() fails with the error EINVAL.  It would be more consistent
       if this case successfully created a stream that then returned end-of-
       file on the first attempt at reading; since version 2.22, the glibc
       implementation provides that behavior.

       In versions of glibc before 2.22, specifying append mode ("a" or
       "a+") for fmemopen() sets the initial buffer position to the first
       null byte, but (if the current position is reset to a location other
       than the end of the stream) does not force subsequent writes to
       append at the end of the stream.  This bug is fixed in glibc 2.22.

       In versions of glibc before 2.22, if the mode argument to fmemopen()
       specifies append ("a" or "a+"), and the size argument does not cover
       a null byte in buf, then, according to POSIX.1-2008, the initial
       buffer position should be set to the next byte after the end of the
       buffer.  However, in this case the glibc fmemopen() sets the buffer
       position to -1.  This bug is fixed in glibc 2.22.

       In versions of glibc before 2.22, when a call to fseek(3) with a
       whence value of SEEK_END was performed on a stream created by
       fmemopen(), the offset was subtracted from the end-of-stream
       position, instead of being added.  This bug is fixed in glibc 2.22.

       The glibc 2.9 addition of "binary" mode for fmemopen() silently
       changed the ABI: previously, fmemopen() ignored 'b' in mode.

EXAMPLES         top

       The program below uses fmemopen() to open an input buffer, and
       open_memstream(3) to open a dynamically sized output buffer.  The
       program scans its input string (taken from the program's first
       command-line argument) reading integers, and writes the squares of
       these integers to the output buffer.  An example of the output
       produced by this program is the following:

           $ ./a.out '1 23 43'
           size=11; ptr=1 529 1849

   Program source

       #define _GNU_SOURCE
       #include <string.h>
       #include <stdio.h>
       #include <stdlib.h>

       #define handle_error(msg) \
           do { perror(msg); exit(EXIT_FAILURE); } while (0)

       main(int argc, char *argv[])
           FILE *out, *in;
           int v, s;
           size_t size;
           char *ptr;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s '<num>...'\n", argv[0]);

           in = fmemopen(argv[1], strlen(argv[1]), "r");
           if (in == NULL)

           out = open_memstream(&ptr, &size);
           if (out == NULL)

           for (;;) {
               s = fscanf(in, "%d", &v);
               if (s <= 0)

               s = fprintf(out, "%d ", v * v);
               if (s == -1)


           printf("size=%zu; ptr=%s\n", size, ptr);


SEE ALSO         top

       fopen(3), fopencookie(3), open_memstream(3)

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

GNU                              2020-04-11                      FMEMOPEN(3)

Pages that refer to this page: fdopen(3)fopen(3)fopencookie(3)freopen(3)open_memstream(3)open_wmemstream(3)